The invention relates to a high resolution objective for stereomicroscopes of the telescope type which contains three optical assemblies, the first assembly being arranged towards the object end and the third assembly being arranged towards a magnification changer.
Stereomicroscopes are usually equipped with a magnification changer which on the one hand allows high magnification of the object and on the other hand makes it possible to view large object fields. These devices are used for example in technology, for manipulating and inspecting small objects such as, for example, semiconductor structures or micromechanical objects, in research institutes involved in biosciences and materials science and also, for example, for examining and manipulating cells or for surgical purposes. In the course of miniaturising and researching into ever smaller preparations, the requirements imposed on the resolution of these microscopes are increasing and on the other hand the size of the field of vision becomes more important with less magnification for rapid positioning of the slides and for improving the overall view in observations.
In order to vary the magnification of a stereomicroscope magnification changers (the telescopic or zoom principle) are inserted behind the objective. The ratio of maximum to minimum magnification is designated z. Using a zoom the magnification can be varied smoothly over a particular range. Afocal zooms are known which image object rays from infinity to infinity and allow the magnification to be varied without altering the position of the object and the image.
FIG. 1 shows the basic structure of a stereomicroscope of the telescope type. The stereomicroscope allows the user, whose eyes are indicated as 8R and 8L, to obtain a three-dimensional impression of the object 1 being observed. The object 1, which is in the front focal point of the objective 2, is imaged using two separate optical channels. The two viewing channels 10L and 10R are of similar construction and each contain a magnification changer system 3L, 3R, a tube lens 4L, 4R and an eyepiece 7L, 7R. Image reversing systems 5L, 5R mounted behind the tube lenses 4L, 4R provide upright intermediate images 6L, 6R on the correct side which are visually viewed using the pair of identical eyepieces 7L, 7R. These pairs of optical elements are arranged parallel and symmetrical to the axis of the objective 2. The two magnification changers 3L, 3R alter the magnification selectively but in the same way for the left and right hand channels 10L, 10R.
The two intermediate images 6L and 6R are different images of the object 1 as the object 1 is viewed at an angle wL in the left hand channel 10L and at an angle wR in the right hand channel 10R. In this way it is possible to view the object 1 stereoscopically in the same way as when looking directly at an object through the eyes 8L, 8R. The two different images are processed in the brain to form a three-dimensional image impression.
EP denotes the diameter of the entry pupil of the magnification changers 3L, 3R which are adjustable in the same way. uL and uR denote the half aperture angles of the cone with the vertex in the centre of the object OM, which is bounded by the entry pupil. uL and uR are the same size, as the microscope is symmetrical with respect to the axis 9 of the objective 2. Consequently, uL and uR may both be referred to as u. As wR and wL are not large, the equation EP=2xc3x97Fxc3x97sin(u)=2xc3x97Fxc3x97nA applies here, where nA is the effective numerical aperture (in air) of the objective, based on the entry pupil of the magnification changer 3L or 3R downstream of the objective 2 and F is the focal length of the objective 2, this equation being known for well corrected optical systems which satisfy the sine condition. With a wavelength of xcex=550 nm the resolution capacity is 3000xc3x97nA as a rule of thumb (in pairs of lines per mm). Thus, a high aperture is a prerequisite for a high resolution.
At the maximum magnification of the magnification changer 3L, 3R the entry pupil diameter EP is at its maximum and is then known as ENP. The magnification changers shown may be the afocal zooms or telescope systems mentioned earlier.
FIG. 1 also shows the path of a ray running from the lower edge of the object Ou to the edge of the intermediate image. It forms an angle w in the space between the objective and the magnification changer with the axis 9 of the objective 2. w is the field angle of the objective 2, which is at its maximum when the magnification of the magnification changer 3L and 3R is at its weakest. The maximum value of w is hereinafter referred to as w1. The object width, i.e. the spacing of the object 1 from the first surface of the objective 2, is designated OW.
FIG. 1 shows the objective 2 purely diagrammatically. As a rule, the objectives form lens systems consisting of individual lenses and/or cemented lenses (cemented members).
Published Japanese Application JP 2001-147378 discloses an objective system for a stereomicroscope which consists of three sets of lenses, the two outer sets of lenses having a positive refractive power while the central set of lenses comprises at least one cemented member consisting of three lens elements. In one embodiment by way of example, the lens assembly at the object ends consists of a single lens followed by the second lens assembly which contains two cemented members having three or two cemented lenses, which is in turn followed by the third lens assembly consisting of a cemented member with two lenses. This structure is intended to suppress the optical imaging errors, distortion and chromatic aberration as far as possible. The embodiments published have a numerical aperture nA of 0.13 and 0.20. The resolution of these objectives is thus within the known range.
Published Japanese Application JP 101 70 832 A describes a conversion lens which can be placed in front of the objective in a stereomicroscope and in conjunction with this main objective produces a short focal length. As a result of the high magnification produced, the conversion lens is used for viewing extremely small object structures while in order to observe the object in a large field of vision the conversion lens can be removed. The disadvantage of such a construction is the high number of lenses used and the need to install or remove a conversion lens. The proposed combination consists of a total of twelve lenses. The conversion lens in the publication referred to consists of three lens assemblies of which the two outermost have positive refractive power and each consist of two cemented lenses while the central lens assembly is a cemented component in the shape of a meniscus. The actual main objective, viewed from the object end, consists of a single lens followed by a cemented member consisting of two lenses, followed by another cemented member comprising two lenses and another single lens.
Olympus provide a conversion lens of this kind under model reference SZX-AL20. The combination of conversion lens and objective has a numerical aperture of nA=0.275 but can only be used in a limited range of the zoom, outside which the image is cut by vignetting. The object spacing OW of the combination of objective and conversion lens is only about 10 mm, thus making it substantially more difficult to work with objects under the microscope.
The present applicants have marketed a stereomicroscope under model reference xe2x80x9cMZ 12xe2x80x9d for which objectives with a focal length of F=50 mm and a maximum entry pupil diameter of ENP=20 mm are available. In this objective, which already has a high magnification, the spacing of the object from the first surface of the objective OW=21.3 mm, making it possible to work with objects under acceptable conditions.
Finally, U.S. Pat. No. 4,640,586 discloses an objective for a stereomicroscope of the telescope type which helps to avoid optical illusions with regard to flatness when examining an object. In one embodiment (Example 5) the objective consists of two individual lenses with positive refractive power followed by a meniscus-shaped cemented member followed by another meniscus-shaped cemented member and a third individual lens with positive refractive power. The focal length is 49.98 mm, the spacing of the axis of the magnification changers is given as 22 mm, which means that the maximum diameter of the circular entry pupil of the magnification changer must be a little less than 22 mm.
The aim of the present invention is to provide an improved objective of the type specified which when used in a stereomicroscope with a magnification changer has a high resolution with powerful magnification adapted thereto, the resolution capacity being higher than that of known stereomicroscopes, and wherein furthermore the entire range of magnifications of the microscope should be useable without any vignetting. At the same time the microscope should have the largest possible working gap.
This problem is solved by an objective according to the main claim. Some advantageous embodiments will become apparent from the subsidiary claims and the description that follows.
According to the invention the objective for stereomicroscopes of the telescopic type with a magnification changer meets two conditions, namely                     0.44        ≤                  ENP          F                ≤        0.6                            (        B1        )            
at the maximum microscope magnification and
(B2) 0.16xe2x89xa6tan(w1) at the lowest microscope magnification.
Condition B1 provides a lower and an upper limit for the half aperture angle u (see FIG. 1) of the cone of rays with its vertex in the centre of the object OM, which is defined by the entry pupil at the maximum magnification. The sine of the aperture angle u corresponds to the numerical aperture nA of the stereomicroscope. As the resolution capacity is furthermore proportional to the numerical aperture nA of the stereomicroscope, the condition B1 constitutes a lower and upper limit for the resolution. The condition B1 links the focal length of the objective to the maximum diameter of the entry pupil of the magnification changer and ensures that the maximum microscope magnification is adapted to the resolution of the microscope and that no empty magnification is produced if, as usual, only the optical system comprising the magnification changer, tube and eyepiece is working within the range of the useful magnification.
Condition B2 provides a lower limit for the field angle w of the objective (see FIG. 1) at the weakest magnification of the microscope. Meeting this condition guarantees that in microscopes in which the ratio z of maximum to minimum magnification is large, even at the lowest magnification the entire field of vision theoretically obtained can be used without any vignetting.
Stereomicroscope objectives of the kind mentioned which satisfy conditions B1 and B2 differ from those of the prior art in that they have a higher resolution and can be used throughout the range of magnifications of the microscope (without the additional use of conversion lenses or the like and without any vignetting).
If, for example, a zoom with a zoom factor zxe2x89xa712 and an eyepiece with a visual field number of xe2x89xa721 mm is used as the magnification changer, in stereomicroscopes operating in the range of useful magnification, it is no longer possible to use the entire field of vision theoretically produced if at the lowest magnification it falls below the lower limit set by condition B2.
It is particularly advantageous to set the lower limit of condition B1 as 0.55 to ensure a significant increase in the resolution of the objective. Embodiments of objectives are given hereinafter by way of example satisfying the conditions specified above.
In the objective according to the invention which consists of three optical assemblies, further conditions for the first assembly G1 facing the object and for the third assembly G3 remote from the object have proved advantageous, while the following equation should apply to the focal length f1 of the first assembly:       1.3     less than           f1      F         less than     1.8    ,
while this first assembly in particular consists of a single lens, and the following equation       2     less than           f3      F         less than     4    ,
should apply to the focal length f3 of the third assembly, and the third assembly in particular also consists of only a single lens.
A shorter focal length of the first optical assembly (in other words f1 less than 1.3xc3x97F) in the embodiments of the objective according to the invention results in greater aberrations at the edge of the image which are difficult to compensate for in the subsequent assemblies G2 and G3. If on the other hand the focal length f1 exceeds a value of 1.8xc3x97F this results in a larger objective diameter which results in undesirably high weight. All in all, it has proved advantageous to construct the first assembly G1 as an individual lens.
Even though only parts of the aperture of the objective are used during stereo viewing in each channel, it is nevertheless essential to have good correction of the aperture error of the objective with the full aperture. This is best done on the third optical assembly of the objective as the beam diameter is great at this point. A focal length f3 corresponding to 2 to 4 times the focal length of the objective as a whole has proved advantageous. A longer or shorter focal length is unsuitable for compensating the aperture error which is not necessarily fully corrected in the assemblies G1 and G2. Moreover, a shorter focal length f3 makes it more difficult to construct the assembly G1 and particularly the assembly G2 if a substantial working distance OW is required. It has been found that the optical assembly G3 can advantageously be produced as a single lens.
The term       OW    F    ≥  0.42
correlates the spacing of the object
plane from the first surface of the assembly G1 of the objective according to the invention with the total focal length F of this objective and guarantees an object distance which allows the user to work comfortably underneath the microscope. With a preferred objective focal length of F=40 mm, OW must be xe2x89xa716.8 mm.
With regard to the second optical assembly G2 it has proved favourable if the last radius Rk of this second group pointing towards the group G3 satisfies the term   0.7   less than       Rk    F     less than       1.1    .  
In this given range it is possible, in
particular, to adapt the assemblies G2 and G3 to one another in such a way that for objectives with high apertures and large fields of this kind it is possible to correct the imaging errors with a small number of lenses. A construction in which the three optical assemblies consist of a total of not more than 8 lenses is possible and beneficial, with a high imaging quality, as the costs and weight of the stereomicroscope go up as the number of lenses increases.
High quality correction of chromatic aberrations can be achieved by a suitable choice of optical glass which satisfies the conditions
|Pg,Fxe2x88x920.6438+0.001682xc3x97xcexdd| greater than 0.0075 
or
|PC,txe2x88x920.5450xe2x88x920.004743xc3x97xcexdd| greater than 0.025, 
where xcexdd=(ndxe2x88x921)/(nFxe2x88x92nC) is the Abbe coefficient, Pg,F=(ngxe2x88x92nF)/(nFxe2x88x92nC) denotes the relative partial dispersion for the wavelengths g and F and PC, t=(nCxe2x88x92nt)/(nFxe2x88x92nC) indicates the relative partial dispersion for the wavelengths C and t and n denotes the refractive index at the wavelength in question.
With lenses of this kind it is possible not only to correct the primary chromatic aberrations but also to reduce the secondary spectrum significantly. It is particularly advantageous to use lenses of this kind for the optical assembly G3 which consists in particular of a single lens.
A further advantage arises from the use of at most three different materials, particularly the above mentioned chromatic aberration correcting glass, for the optical assemblies of the objective according to the invention.